For circuits such as switching power supply circuits and inverter circuits, semiconductor devices such as transistors and diodes are used. It is demanded that these semiconductor devices have a high breakdown voltage and low on-resistance. Furthermore, a relationship between a breakdown voltage and the on-resistance is a tradeoff relationship determined according to a semiconductor material.
Due to an advancement of technology, the high breakdown voltage and the low on-resistance of the semiconductor devices has become closer to a limit of potential of silicon that is a major semiconductor material. It is necessary to change a semiconductor material to further improve the breakdown voltage and further reduce the on-resistance. By using a nitride semiconductor such as a gallium nitride or an aluminum gallium nitride as a semiconductor material of semiconductor devices, it is possible to improve a tradeoff relationship determined according to the semiconductor material. Consequently, it is possible to realize higher breakdown voltage and lower on-resistance of semiconductor devices.
A transistor for which a nitride semiconductor is used generally adopts an HEMT (High Electron Mobility Transistor) structure that uses a two dimensional electron gas (2DEG) as a carrier. A conventional HEMT is a normally on transistor that is conducted without applying a voltage to a gate. Therefore, it is difficult to realize a normally off transistor that is not conducted unless a voltage is applied to a gate.
By applying to the HEMT a gate recess structure that a gate electrode is provided in a trench (recess), it is possible to increase a threshold voltage and realize the normally off transistor. It is possible to reduce a two dimensional electron gas density under a gate electrode by providing the gate electrode in a trench or increase the threshold voltage of the HEMT by extinguishing the two dimensional electron gas.